Many studies have shown that both household and business consumers can dramatically reduce their power consumption if they can easily and continuously monitor their power usage. Estimates range from 10 to 20 percent savings for a typical household. In addition, many businesses and household consumers are strongly motivated to save energy so as to reduce consumption of hydrocarbons and reduce carbon emissions by reducing the amount of power that must be generated by electricity providers using coal and other hydrocarbon energy sources.
Monitoring of power usage can also be critical to optimizing the efficiency and lowering the energy costs of telecommunication central offices and computer data centers.
As is well known in the field, the electrical power being carried by a conductor is the mathematical product of the voltage on the conductor and the current passing through it. It is generally easy to measure voltage in a system via a contacting means. For example, in a typical building electrical power system voltage can be measured at nearly any electrical outlet, since it is generally the same at all points in the system.
However, it can be very difficult to measure the current in a conductor, especially if the current of interest is carried by a conductor which is physically enclosed or shielded within a conduit, or within a wall or junction panel which can only be safely accessed by trained personnel, possibly only after the power has been interrupted.
Electrical current can be measured either invasively, i.e. by directly connecting a sensor in series with the circuit, or non-invasively by sensing the magnetic and/or electric field generated by current in the immediate vicinity of a conductor. FIG. 1 illustrates a current (or voltage) source 100 transmitting an electrical current via a conductor 102 through a load impedance 104, whereby a magnetic field 106 is generated surrounding the conductor 102. This magnetic field 106 can be detected by any of several types of sensor, including a loop of wire or a “Hall-effect” sensor.
In many applications, including home electrical usage, it is generally desirable to measure current non-invasively so as to avoid the need to make direct contact with conductors carrying dangerous levels of voltage. Current can be non-invasively sensed by surrounding a conductor with a “loop” sensor. This approach is relatively insensitive to the precise location and orientation of the sensing loop, and it is relatively insensitive to noise sources such as fields generated by current flowing outside of the loop. However, loop sensors can be difficult to implement if access to the conductor is restricted. Also, periodic calibration can be required, due to long term drift and degradation of the device, as well as variations in temperature and other environmental conditions.
Current can be non-invasively sensed without surrounding a conductor with a sensor, simply by placing a sensor at a location near the conductor. However, the signal obtained from such a sensor will depend strongly on its exact location and orientation relative to the conductor. Also, the signal obtained from such a sensor will include contributions from magnetic fields generated by other nearby sources, such current carried by wires which are physically near the conductor of interest. These effects can make the output of a remote, non-loop sensor difficult to interpret with accuracy.
Typically available non-invasive devices for measuring current are based on the surrounding loop approach, and therefore require physical access to the individual conductors which are to be measured. This can require access to the interior of an electrical panel (such as a circuit breaker panel) and/or invasive intrusion into a service conduit or cable, such as a so-called “service entrance” or SE cable, so as to remove the conductors from the conduit or cable, separate the individual conductors apart from each other, and place a loop around each individual conductor or cable.
Access to conductors within a circuit breaker panel typically requires removal of a circuit breaker panel cover, which can be dangerous and may require the services of a trained professional and the involvement of the power utility company, as well as temporary interruption of electrical service to the building so as to satisfy safety requirements and/or local laws. And once sensors have been installed, this process may need to be repeated periodically for temporary removal and calibration of the sensors, leading to repeated service calls by trained professionals and repeated interruptions in electrical service.
Non-loop devices exist for the non-intrusive measurement of current in cables or conduits containing multiple conductors, such as a service entrance (SE) cable. However, these devices typically are configured for use only with a specific type of cable, and must be accurately aligned with the conductors inside of the cable. Even when properly used, these devices tend to provide inaccurate or unreliable measurements.
Another approach to non-invasive measurement of whole-building current usage is to mount an optical sensor or radio receiver directly on an electrical meter provided by the power company. However, this approach is compatible only with certain types of electrical meters, and therefore cannot be used in many circumstances. Also, this approach cannot be used to monitor current usage on individual branches of electrical service within the building.
In a few regions, the local power company offers a “smart meter” which is installed by the power company in place of the standard electrical meter, and which communicates power usage to the power company on a periodic basis, typically hourly. The power company makes this information available to the consumer over the internet or some other network, thereby allowing the consumer to monitor power usage. Similar meters are available in some areas which transmit their readings to monitoring devices within the building. However, these approaches only provide periodic measurements, and are only available in a few areas. In addition, this approach typically cannot be used to monitor current usage on individual branches of electrical service within the building.
What is needed, therefore, is an apparatus which can accurately and non-invasively monitor current usage without requiring access to the interior of a power distribution cable or a junction box, and without requiring installation of the apparatus by an electrician or other technically knowledgeable individual.